Know your limits: Modelling consumer-resource interactions to derive nutrient thresholds for a sustainable Anthropocene

In the Anthropocene, ecosystems have been strongly influenced by human beings. Eutrophication, or the enrichment with nutrients, is one of the major challenges in the Anthropocene and has become one of the important reasons for the collapse of ecosystems on earth. The collapse of aquatic ecosystems caused by eutrophication is iconic. Human activities such as agriculture, industrial production and domestic consumption discharge nutrients to aquatic ecosystems, causing severe deterioration of aquatic ecosystems when nutrient thresholds are exceeded. This deterioration of the aquatic ecosystem is often accompanied by harmful algal blooms. Toxic algal blooms threaten the safety of drinking water and damage other organisms in the aquatic ecosystem through their shading of light or the depletion of oxygen during decomposition. Thus, effective solutions for eutrophication management are urgently required for a sustainable supply of human needs while preserving ecological functions.

Nutrient thresholds can provide a reference for nutrient control to reduce eutrophication. Nutrient thresholds are often determined based on the load-response curve; i.e., the response of phytoplankton biomass (e.g. represented by chlorophyll-a) to a range of nutrient loadings (e.g. P load). However, it is challenging to determine these curves because of the variability and complexity of biogeochemical processes among aquatic ecosystems. To deal with the variability and complexity of ecosystems, consumer-resource interactions can be used to capture the fundamental mechanisms underlying the relationship between phytoplankton and nutrients, i.e. the load-response curve. In this thesis, the main question I asked is how the consumer-resource interactions can be used to derive nutrient thresholds and help nutrient management towards a sustainable Anthropocene? This question gives rise to a number of sub-questions:

•  how does consumer-resource interaction support the interpretation of the concept of Planetary Boundaries in a global context? (Chapter 2)

•   how can consumer-resource interaction be used to link lab-, field- and model-based knowledge into one approach to shape the load-response curve of aquatic ecosystems with variable characteristics? (Chapter 3)

•  how can consumer-resource interaction be applied to simulate the shift in nutrient limitations in aquatic ecosystems taking into account the stoichiometric variance of the phytoplankton communities? (Chapter 4)

• How do cyanobacterial traits affect nutrient thresholds through consumer-resource interactions in a complex process-based model? (Chapter 5)

To answer these questions, my PhD thesis provides mechanistic frameworks for the load-response curves so that they are adaptive to the structural changes of systems (e.g. lake depths, changing community composition, new species traits). In this thesis, I have presented four different models that are capable of predicting limits such as nutrient thresholds. Three of these models are newly developed within the context of this thesis. Using these, and other existing models I believe we can guide policy and management effectively towards a more sustainable world where targets are based on mechanistic knowledge of our ecosystems. Notably, the RPCW model derived in chapter 2 can be used as didactic tools to show how reducing the consumption rate impacts global limits. In chapter 3, the GPLake model has the potential to be used as a first estimation of chlorophyll-a production under varying P loads and is applicable to a wide range of lakes with different characteristics due to its mechanistic basis. In chapter 4, the GPLake-S captures the variability of phytoplankton community stoichiometry while being simple enough to be easily applicable in eutrophication management. In chapter 5, climate change scenarios can be simulated to see its effect on cyanobacterial traits that affect critical nutrient loadings.

All these different models addressed in this thesis can help to set limits and develop measures aimed towards a sustainable Anthropocene. Models that are able to improve our understanding of the limits to human pollution are essential to safeguard a sustainable future. I have developed models and modelling strategies of varying complexity that all serve this goal. Through such model diversity that can be used separately, or in association with each other, I hope to make a contribution to both science and society.